1. Field of the Invention
This invention relates to a method for fabricating a photodiode, and more particularly, a method for forming a metallic reflecting layer for enhancing the sensitivity of a semiconductor photodiode.
2. Description of Related Art
A photodiode is a device that converts photo energy into an electrical signal through a P-N junction metal-oxide-semiconductor ("MOS") device. When a light ("photon") with sufficient energy is beamed onto a photodiode, the light energy generates electron-hole pairs in the P region and the N region of the P-N junction MOS device. When an electrical field is applied to both ends of a photodiode, the electron-hole pairs flow toward the P-N junction, specifically, the electrons flow toward the N region and the holes flow toward the P region. The congregated electrons and holes at the P-N junction generate electrical signals.
As a matter of industry practice, a commercial image sensor such as a charge-couple device (CCD) may contain a number of MOS photodiodes. Due to the fact that it is difficult to manufacture consistent sizes for MOS diodes and the high cost associated with manufacturing MOS diodes, and that complementary metal-oxide-semiconductor ("CMOS") has shown to be easier to manufacture consistent sizes and lower cost, therefore, the current industry practice is that CMOS photodiodes have been used to replace MOS photodiodes. Additionally, comparing to MOS photodiodes, CMOS photodiodes consume less power. Thus, the usage of CMOS photodiodes is attractive in imaging sensing devices.
FIG. 1 is a schematic cross-sectional view showing the structure of a conventional CMOS photodiode.
Referring to FIG. 1, a conventional CMOS photodiode consists of a gate 106, a first source/drain region 112, and a second source/drain region 113. The gate 106 is formed on a gate oxide layer 105 over a P-well 101 of a substrate 100, wherein the gate 106 is surrounded by a first spacer 108 and a second spacer 107. There is a first lightly doped drain (LDD) 109 formed beneath the first spacer 107, and neighboring to the first source/drain region 112. Similarly, there is a second LDD 110 formed beneath the second spacer 108, and neighboring to the first source/drain region 113. The second source/drain region 113 is used as a depletion region. The foregoing structure is formed within an active region 104 partitioned by isolations 103 and 103.
When photons pass through the transparent gate oxide layer 105 and reach the depletion region 113, electrons and holes generated by the photoelectric effect form a electrical signal at the P-N junction of the aforesaid CMOS photodiode. It is common knowledge in the semiconductor physics that when certain amount of photons is beamed at a photodiode, the sensitivity of each photodiode directly relates to the number of photons reaching the depletion region. Typically, it had been an industry practice for the purpose of enhancing the sensitivity of a photodiode, increasing the top surface area of the depletion region allowing more photons to arrive at the depletion region. However, as CMOS photodiodes are becoming more integrated on a piece of semiconductor, the aforesaid method of increasing the sensitivity of a photodiode by extending the top surface area of the depletion region has become more unpractical.